Death toll includes 20 children in Syrian government attack on besieged enclave of Eastern Ghouta, war monitor says.

At least 77 civilians have been killed near Damascus in less than 24 hours in heavy bombardment by Syrian government forces as they prepare a ground operation to recapture the opposition-held enclave, a war monitor group said.

Air raids and artillery fired on Eastern Ghouta - a suburb of the Syrian capital - have killed at least 20 children since Sunday, the UK-based Syrian Observatory for Human Rights (SOHR) reported.

Some 300 people have been wounded during the attacks, according to the monitor.

Government forces are preparing to launch a major ground assault on Eastern Ghouta following the heavy bombardment, which began on Sunday, SOHR said.

Government planes are "shooting everything that moves inside the residential areas", a local doctor told dpa.

"Our hospitals are overcrowded with wounded. We are running out of anaesthetics and other essential medications," he said.

Activists put the number of dead at 68. "Each minute between 20-30 shells are falling on the residential areas, especially in Hammouriyeh and Sabka," Mazen al-Shami was quoted as saying from the enclave.

The main opposition National Coalition, which is based in Turkey, denounced the "war of extermination" in Eastern Ghouta as well as the "international silence".

In a statement, it also accused Syria's ally Russia of seeking to "bury the political process" for a solution to the conflict.

Eastern Ghouta, the last remaining rebel-held area near Damascus, has been under siege by government forces since 2013. It is home to about 400,000 people.

An international aid convoy was able to deliver much-needed food and medical supplies to the enclave on February 14.